1. Field of the Invention
The present invention relates to the mining of sulfur and to the production of sulfuric acid. More particularly, it relates to the integration of the production of sulfuric acid by the contact process and the mining of sulfur from underground deposits by the Frasch process.
2. Description of the Prior Art
Both the contact process for manufacturing sulfuric acid and the Frasch process for mining sulfur are well known to those skilled in these arts, and descriptions of their operations may be found in the patent literature and in numerous chemistry books and encyclopedias including, for example, the Kirk-Othmer Encyclopedia of Chemical Technology, Second Edition, Vol. 19, pp. 337-348 and 460-482, John Wiley & Sons, Inc., 1969. In the contact process for producing sulfuric acid, sulfur dioxide (SO.sub.2) produced, for example, from the burning of elemental sulfur, is converted to sulfur trioxide (SO.sub.3) with the aid of a catalyst such a vanadium pentoxide, and the SO.sub.3 is absorbed in dilute sulfuric acid to produce a strong, hot product acid which is then cooled prior to storage and shipment for use. In the Frasch process for mining sulfur, a hot aqueous mining fluid, e.g., water, is used to melt the solid sulfur present in an underground sulfur-bearing formation by injecting the fluid, heated under pressure to around 325.degree. F., through the annulus formed by two concentric pipes and using compressed air to lift the molten sulfur to the surface through the center pipe. The air is usually forced down through a small diameter pipe located within the described concentric arrangement.
Until recent years, the source of heat for the operation of a Frasch process sulfur mine has been the relatively abundant, low-cost supply of natural gas. However, as these reserves dwindle and gas supplies, when available, soar in price, it is becoming increasingly necessary to resort to the use of other fuels such as oil or coal. The use of either of these latter materials, though presently readily available, is also extremely costly. The price of imported oil has risen drastically and its continued availability is subject to question and concern. Coal is readily available domestically but it is very costly, if even possible, to convert existing gas-fired boilers to those firing coal and oil. The presence of sulfur as a contaminant in the oil and coal also presents disadvantages from the standpoint of possible air pollution, as well as from the standpoint of water-treating problems in those mining plants wherein the combustion gases from the boilers directly contact the incoming cold mining fluid. In the latter situation, the SO.sub.2 formed is absorbed by the water and may reduce the pH to the corrosive range and require the corrective addition of costly soda ash, or the concentration of sulfite in the water may increase to the point at which excessive calcium sulfite scaling occurs in heat exchangers and other water-handling equipment of the facility.